The instant invention relates to accuracy testing devices or gages for testing the accuracy of machine tools, including computer controlled robots, and more particularly to a sliding ball bar test gage operable for long range linear measurements, as well as circular or spherical measurements at infinitely adjustable radii.
Telescoping ball bar test gage devices for circular or spherical (out of roundness) testing of machine tools have heretofore been known in the art. In this regard, the U.S. Pat. Nos. to Bryan No. 4,435,905 and Burdekin No. 5,052,115 represent the closest prior art to the subject invention of which the applicant is aware. The patent to Bryan discloses a telescoping magnetic ball bar test gage for testing the accuracy of computer controlled machine tools. Two gage balls are held and separated from each other by a fixture which allows slight telescoping movement but not lateral movement. The telescoping fixture comprises a parallel reed flexure unit and a rigid bar. The telescoping fixture is really only operative for telescoping movement of a small fraction of an inch as provided for by a telescoping LVDT in the parallel reed flexure unit. One gage ball is secured by a magnetic knuckle socket assembly fixed to a central point on the work table of the machine to be tested. The other gage ball is secured by another magnetic knuckle socket assembly which is attached to the active movable element of the machine. The machine is then programmed in such a manner that the center of the gage ball on the active element of the machine is directed to execute a prescribed trajectory, all points being equidistant from the center of the fixed gage ball. As the moving ball executes its trajectory, changes in radial distance between the centers of the two balls caused by inaccuracies in the machine are determined by a linear variable differential transformer (LVDT) assembly actuated by the parallel reed flexure unit. The rigid bar of the telescoping unit is provided with extensions in order to test accuracy at different fixed size circular trajectories. The patent to Burdekin discloses another machine tool testing device comprising a first ball probe adapted to be fixed to the work table of the machine tool and a second ball probe adapted to be attached to the spindle of the machine tool. The testing device further includes a rigid fixture for holding the ball probes in spaced linear relation and for measuring the distance between the centers of the two ball probes as the second ball probe is moved in a circle around the first ball probe. The fixture comprises four round rods which are held in a square array by two spacers having holes therein for receiving the rods. At one end of the rods is a socket assembly for engagement with the fixed ball probe on the work table. The other end of the rods extend outwardly from a spacer wherein they are operable sliding engagement over the movable ball probe on the active element. A single axis transducer is mounted in the spacer adjacent the free ends of the rods for engagement with the movable ball probe. The machine tool is programmed such that the movable ball probe is moved in a circular trajectory around the fixed ball probe. As the probe is moved, the transducer measures slight changes in the distance between the centers of the two ball probes caused by inaccuracies in the machine tool.
While the above devices are effective for measuring changes in radial distance between two ball probes as one ball probe is moved relative to the other fixed ball probe, they have several drawbacks which have prevented their widespread marketing and use. The first drawback is their limited utility. Both the Bryan and Burdekin devices are only operable for circular testing at fixed radii. The fixtures of both Bryan and Burdekin are of fixed length and the measurement devices are operative for measuring only small changes in the radial distance between the ball as they are moved relative to one another. In order to test at several different radii in the Bryan device, additional links need to be added to the fixture. In order to test at several different radii in the Burdekin device, a different length fixture must be utilized. It can thus be seen that testing procedures at several different radii requires disassembly and reassembly of the device for each test.
Another drawback is that the Bryan and Burdekin devices are not operable for linear testing. When testing machine tools for accuracy, many different types of tests must be conducted in order to ensure accuracy in all dimensions. In addition to circular or spherical tests, linear tests are also performed to determine volume, distance, velocity, squareness, and straightness. Linear and volumetric testing are usually accomplished by means of a laser measurement system in which various precision laser optics are set up on the work table of the device and a laser head is employed to fire a laser beam through the optics wherein various linear measurements are obtained. The LVDT of Bryan, and the single axis transducer of Burdekin are only operable for measuring distances in fractions of an inch and thus are not useful for linear testing.
Yet another drawback to all the prior art devices, including the laser measurement system, is that they are extremely difficult to handle, and difficult to set up for testing. Because the devices are difficult to handle, the testing procedures are time consuming, thus tying up expensive machines for unknown periods of time. It can thus be seen that the prior art testing devices are neither cost effective nor time efficient for regular periodic testing, maintenance, and calibration of machine tools.
The instant invention provides a long range linear test gage which is operable for out of roundness measurements, i.e. circular and spherical tests, as well as longer range linear and volumetric tests. The instant test gage comprises an elongated linear guide assembly, a first reference element defining a first locus, a second reference element defining a second locus, and a linear measuring device capable of measuring long distances at highly accurate tolerances. The linear guide assembly includes an elongated guide profile, a guide rail secured to the guide profile, and a slide which is slidably movable relative to the guide rail between the first and second ends of the guide profile. The first reference element is attached to the first end of the guide profile and the second reference element is attached to the slide. A first magnetic knuckle or socket assembly is provided for securing the first reference element in a fixed position relative to the machine tool. A second magnetic knuckle or socket assembly is provided for securing the second reference element to an active element of said machine tool. During testing, the active machine element is operative for moving the second reference element along a predetermined trajectory, i.e. circular, spherical or linear. The linear guide assembly permits co-linear movement of the reference elements as the active machine tool element moves the second reference element along a predetermined trajectory. The linear measuring device comprises a linear encoder in a first embodiment. The linear encoder includes a scannable measurement index affixed to the guide profile and a scanning head secured to the slide for scanning the index as the second reference element is moved along its predetermined trajectory. In another embodiment, the linear measurement device comprises a laser measurement system wherein a linear interferometer is mounted to the slide and a linear reflector is mounted adjacent the first end of the guide profile. A laser head is provided for firing a laser beam through the linear interferometer either directly or through fiber optics and down the length of the guide profile. A laser detector is mounted to the slide for reading the reflected laser beam wherein linear measurements are read. The test gage further includes a counterweight for counterbalancing the weight of the guide system and measurement device. The test gage still further includes anti-rotation apparatus for preventing rotation or swinging movement of the test gage during use. The instant measuring device is simple to operate and significantly reduces test time. The device is also much more cost effective than the prior art devices.
Accordingly, it is an object of the instant invention to provide a highly accurate linear test gage for testing the accuracy of machine tools.
It is another object to provide a linear test gage which is inexpensive.
It is another object to provide a test gage which is operable for linear, volumetric and out of roundness testing.
It is another object to provide a universal test gage which is simple to set up for numerous tests and simple to operate.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.